Method and apparatus for making corrugations in tubes consisting of ductile material

ABSTRACT

A tube consisting of ductile metallic or plastic material is provided with circumferentially complete corrugations, helical grooves and/or axially parallel grooves while it travels between two universally movable bearings so that it can flex sideways without any appreciable elongation or hardening of its material. The grooves or corrugations are formed by tools having concave tube-engaging faces and rotatably mounted in a holder which orbits about the tube between the two bearings. The tools can be fixedly or adjustably mounted in the holder; in the latter instance, a single set of tools can be used to form circumferentially complete corrugations, helical grooves or axially parallel grooves.

United States Patent Levacher et al.

METHOD AND APPARATUS FOR MAKING CORRUGATIONS IN TUBES CONSISTING 0F DUCTILE MATERIAL lnventors: Friedrich Levacher, Brauweiler; Hans Leo Dltscheid, Refrath; Walter Burger, Bechen; Peter Asselborn; Werner Crarner,

both of Bergisch Gladbach, all of Germany Felten It Guillenume Knbelwerke Alttiengesellschait, ('ologne'Mulheim. (iermnny Filed: July 13, 1970 Appl. No.: 54,247

Assignee:

Foreign Application Priority Data References Cited UNITED STATES PATENTS 3,464,250 9/1969 Stetka ..72/77 3,543,551 12/1970 Raisch et al.. ..72/77 2,614,607 10/1952 Klein ..72/194 FOREIGN PATENTS OR APPLICATIONS 585,377 10/1959 Canada ..72/77 Primary Examiner-Richard J. Herbst Attorney Michael S. Striker 57 ABSTRACT A tube consisting of ductile metallic or plastic material is provided with circumferentially complete corrugations, helical grooves and/or axially parallel grooves while it travels between two universally movable bearings so that it can flex sideways without any appreciable elongation or hardening of its material. The grooves or corrugations are formed by tools having concave tube-engaging faces and rotatably mounted in a holder which orbits about the tube between the two bearings. The tools can be fixedly or adjustably mounted in the holder; in the latter instance, a single set of tools can be used to form circumferentially complete corrugations helical grooves or axially parallel grooves.

24 Claims, 1 1 Drawing Figures P'ATENTEnJunzv m2 3. 672. l 96 SHEEI 2 or 5 INVENTO RS FRIEDRICH AEV CHEQ HANS LEO 0/ 7756HE/0 WALTER BURGER PE TEE ASSL BOB/V WEE V52 CQAMEQ THE R ATTORNEY PATENTEDJUHN I972 3.672.196

sum 5 0F 5 INVENTORS R EDB/cH LVACHE HANS LEO 0/756H5/0 WALTER 52/2652 PETER ASSELBORN Mex/v52 (tam 52 MJIM THEN? ATTORNEY METHOD AND APPARATUS FOR MAKING CORRUGATIONS IN TUBES CONSISTING OF DUCTILE MATERIAL BACKGROUND OF THE INVENTION The present invention relates to a method and apparatus for the production of corrugated tubes, and more particularly to a method and apparatus for the formation of circumferential, helical and/or other corrugations or grooves in metallic or plastic tubes which consist of ductile material and which can be used, for example, as sheaths for electric cables or the like.

An important function of corrugated sheaths for electric cables is to furnish a satisfactory mechanical and electrical protective action as well as to permit ready flexing of cables. Thus, the sheath should offer little resistance to flexing in any desired direction and it should permit flexing to a greater or lesser extent.

Many presently known apparatus for making corrugations or other types of grooves in metallic or like tubes employ pressing or deforming tools which are caused to travel about the tube to form therein a circumferentially complete or a helical groove. In order to reduce friction between the ductile material of the tube and the material of the tool, the latter often assumes the form of a roll whose deforming action is similar to that of a pipe cutter, i.e., the formation of grooves or corrugations brings about a stretching or elongation and resulting reduction in the wall thicknes of the tube. Such reduction in wall thickness in the region of corrugations in a metallic tube is highly undesirable, especially since it is often accompanied by localized hardening of material. It was found that presently known travelling tools operate satisfactorily only when used for the formation of shallow corrugations in relatively thin-walled tubes.

Certain other types of presently known apparatus employ travelling corrugating wheels including those having a concave profile. Experiments which were carried out with such apparatus indicate that the wheels are not suited for the making of relatively deep corrugations in thick-walled tubes; it was found that such tools, too, cause localized reduction in wall thickness and localized hardening of material of which the tubes consist. On the other hand, relatively deep corrugations are often desirable because they enhance the flexibility of the product.

SUMMARY OF THE INVENTION An object of the invention is to provide a method of forming ductile metallic or plastic tubes with circumferentially complete, helical or other corrugations or grooves without any appreciable localized hardening and/or reduction in wall thickness of tubes.

Another object of the invention is to provide a method which renders it possible to form relatively deep or shallow corrugations in thick-walled large-diameter tubes, for example, in metallic tubes having a diameter exceeding 40 millimeters and a wall thickness which exceeds 4 percent of the diameter.

A further object of the invention is to provide a method of providing tubes consisting of ductile material with corrugations or grooves in such a way that the formation of grooves or corrugations does not result in deformation of non-corrugated portions of the tubes.

An additional object of the invention is to provide an apparatus which can be utilized for the practice of the aboveoutlined method.

Still another object of the invention is to provide the apparatus with novel holders for grooving or corrugating tools.

An ancillary object of the invention is to provide a versatile apparatus which can be converted to form in metallic or plastic tubes circumferentially complete, helical or otherwise oriented grooves or corrugations.

The method of the present invention is employed for forming corrugations or analogous grooves in tubes consisting of ductile material, particularly in metallic or plastic tubes for use as sheaths in electric cables. The method comprises the steps of moving a tube lengthwise along a predetermined path, universally movably guiding two longitudinally spaced portions of the tube so that a section of the tube between such portions can flex sideways without any or without appreciable stretching of the ductile material of the tube, and orbiting about the section a plurality of corrugating or grooving tools in such positions and at such a distance from the path that the section is flexed laterally and grooved by successive tools, alwaysbyone tool atatime.

The tools can be positioned to provide the tube with circumferentially complete, helical and/or axially parallel groov The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved corrugating or grooving apparatus itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic partly elevational and partly sectional view of an apparatus which embodies one form of the invention and is designed to provide metallic or plastic tubes with circumferentially complete corrugatiom;

FIG. 2 is an enlarged fragmentary sectional view as seen in the direction of arrows from the line Il-Il of FIG. I and illustrates one mode of mounting the grooving or corrugating tools in their holder;

FIG. 3 is an enlarged view of a detail in the apparatus of FIG. 1 and further shows one mode of lubricating the tools;

FIG. 4 illustrates a portion of a modified apparatus wherein the mas of the orbiting holder for the tools is balanced by a counterweight;

FIG. 5 is an enlarged fragmentary end elevational view of a grooving or corrugating unit with adjustably mounted tools;

FIG. 6 is a fragmentary partly side elevational and partly sectional view of a grooving or corrugating unit constituting a modification of the unit shown in FIG. 5',

FIG. 7 is a fragmentary schematic side elevational view of still another apparatus wherein the holder for the tools constitutes a planetary;

FIG. 8 is a side elevational view of a portion of a finished tube with identical circumferentially complete corrugations formed therein at difierent distances from each other;

FIG. 9 is a side elevational view of a portion of a finished tube which is formed with groups of closely adjacent identical circumferentially complete corrugations;

FIG. 10 is a side elevational view of a portion of a finished tube having circurnferentially complete corrugations of different depth and shape disposed at different distances from each other; and

FIG. 1 l is a side elevational view of a finished tube which is provided with circumferentially complete corrugations, helical grooves and axially parallel grooves.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. I, there is shown a corrugating or grooving apparatus which comprises a suitable mechanism M for feeding a metallic tube 1 axially in the direction indicated by arrow la. The tube is caused to pass through and is guided by a first universally movable hollow spherical bearing 60 which is mounted in a stationary support 81, and thereupon through a second universally movable hollow spherical bearing 6b mounted in a second support S2. The corrugating or grooving station is located between the bearings 60, 6b and accommodates a wheelshaped tool holder 2 which is rotatable about the axis of a shaft 4 and is caused to orbit circumferentially about the axis of the travelling tube 1. The holder 2 carries a set of equidistant radially extending grooving or corrugating tools 3. The axis of the shaft 4 is normal to the axis of the tube 1. The holder 2 is assumed to rotate about the axis of the shaft 4 in response to lengthwise movement of the tube but it can also be driven by a gear train or the like. The distance between the shaft 4 and the path for the tube 1 is selected in such a way that the tool 3 which engages the tube causes the latter to flex (downwardly, as viewed in FIG. 1) whereby such flexing takes place simultaneously with the formation of circumferential corrugations C. Such flexing of the tube 1 takes place without any elongation because the tube travels through the passages defined therefor by the spherical bearings 60 and 6b. Therefore, the corrugations C are formed without localized reduction in the wall thickness of the tube and without hardening of the metallic material. The spacing between successive tools 3 (as considered in the circumferential direction of the tool holder 2) is such that a tool engages the tube 1 only when the formation of the preceding corrugation C is completed. This is desirable because simultaneous engagement of the tube by two tools 3 would interfere with desirable flow of metallic material of the cylindrical wall 7.

Since the corrugated portion of the tube 1 is more readily flexible than the untreated (left-hand portion), the distance A1 between the center of the right-hand bearing 6b and the axis of the shaft 4 is less than the distance A2 between the axis of the shaft 4 and the center of the left-hand bearing 64. [t was found that the distance A2 should be at least 2.5 times and that the distance A] should be at least 1.5 times the diameter D of the tube 1. The vertical plane including the axis of the shaft 4 intersects the tube 1 in the region where a tool 3 begins to move away from the axis 5 of the tube. The reference character Dw denotes the maximum diameter of the corrugating or grooving unit including the holder 2 and tools 3. The apparatus is preferably furnished with several grooving or corrugating units, each having a different diameter Dw, depending on the diameter of the tube. The tube can consist of any ductile metallic or plastic material.

FIG. 2 shows that each tool 3 comprises a concave work-engaging face 3A and is seated in a socket or notch 11 in the disk-shaped main body portion 2a of the holder 2. The latter has two externally threaded extensions 2b, 2r: which mesh with threaded end walls 12, 13 having suitably configurated conical flanges which engage the inclined end faces of the tools 3 and hold them against movement axially as well as radially of the main portion 2a. The radius R of curvature of the concave face 3A is between 1.1 and 1.8 times the radius R, of the untreated part of the tube 1.

Referring to FIG. 3, the maximum diameter Dw of the grooving or corrugating unit including the holder 2 and tools 3 can be calculated in accordance with the equation wherein .t is the distance between the centers of successive corrugations. t is the maximum depth of a corrugation, r is the radius of curvature of the profile of a tool 3 (this radius equals the desired radius of curvature of surfaces flanking the deepmost portions of corrugations C), and alpha is the angle between the planes of adjoining tools 3 (this angle determines the distance between successive corrugations in the tube).

The above equation and other data pertaining to preferred dimensions and distances are to be considered in the manufacture of various corrugating or grooving units, one for each tube diameter and one for each shape of the corrugation. The number of holders can be reduced if the tools are readily removable and exchangeable in a manner as shown in FIG. 2 or in an analogous fashion.

FIG. 4 illustrates one presently preferred device for orbiting the holder 2 about the axis of the axially moving tube 1. This device comprises a driven carriage S rotatably mounted on the support 51, a bifurcated bracket 51 which is mounted on the carriage 50 and supports the shaft 4 for the holder 2, a similar bifurcated bracket 52 located diametrically opposite the bracket (with reference to the axis of the tube 1 and a counterweight 10 which is rotatably mounted on a shaft 53 supported by the bracket 52. The means for rotating the carriage 50 may comprise a system of gears or the like. The counterweight 10 and its bracket 52 balance the weight of the bracket 51 and corrugating unit 2, 3. The distance between the parts 2, 10 on the one hand and the axis 5 of the travelling tube 1 on the other hand can be adjusted by means of a feed screw 8 driven by an electric motor 9 and meshing with the brackets 51, 52. The motor 9 can be connected with an energy source by means of customary slip rings or the like, not shown. The faces 3A of the tools 3 can be lubricated by way of lubricating ducts 22 (see also FIG. 3) provided in the bodies of such tools and receiving lubricant from the interior of the holder. The ducts 22 may (but need not) be omitted if the lubricating system comprises one or more nozzles 23 mounted on the counterweight l0 and serving to discharge lubricant into the corrugations C of the tube 1. The illustrated nozzle 23 shares the orbital movement of the holder 2 about the axis of the tube. The configuration of the counterweight 10 is similar to that of the holder 2.

It was found that the useful life of tools is much longer if the faces 3A are lubricated in a manner as shown in FIG. 3 or 4. The consumption of lubricant can be kept to a minimum if such lubricant is discharged only through the duct 22 of that tool 3 which engages the travelling tube.

An apparatus which employs the structure of FIG. 4 is especially suited for experimentation. For example, the person in charge might wish to start the motor 9 and to thus move the holder 2 and counterweight 10 toward or away from the axis 5 of the tube 1 in order to determine the optimum distance for the formation of relatively deep, medium deep or shallow circumferential corrugations in a tube having a certain diameter, a certain wall thickness and consisting of a particular ductile metallic or plastic material.

Referring to FIG. 5, the width of recesses or notches ll 1 (as considered in the circumferential direction of the holder 102) may exceed the thickness of the tools 103 so that these tools can swivel about radial axes l5 and to make with the axis 5 of the travelling tube a desired angle beta. The holder 102 comprises a main body portion [020 and two modified end walls l6, 17 which are angularly movable with reference to each other and have concave recesses 14 for the longitudinal ends of the tools 103. By turning the end wall 16 with reference to the end wall 17 and/or vice versa, the person in charge can change the angle beta. This angle can be reduced to zero if the tools 103 are to provide the tube 1 with circumferentially complete corrugations c. If the angle beta exceeds zero, the tools 103 can provide the tube with a helical groove.

FIG. 6 illustrates a holder 202 which constitutes a modification of the holder 102 and which is also provided with means for changing the inclination of tools 203 about radial axes 215. One of the end walls l6, l7 is replaced with a ring-shaped carrier 200 having a cylindrical flange 20b provided with radial bores 200 for the shafis 18 of the tools 203. The axes of the shafts l8 coincide with the respective axes 215. The inner end portion of each shaft 18 carries a pinion 19 which meshes with a ring gear 20A forming part of a disk 20 which is angularly movably coupled to the carrier 20a and is caused to change its angular position when the operator wishes to adjust the inclination of the tools 203. All of these tools are adjustable to the same degee and in the same direction, depending on the direction of rotation of the disk 20. The structure of H6. 6 is preferred in apparatus wherein the angle beta must be varied at frequent intervals. The pinions 19 can be replaced with gear segments and the ring gear 20A may also comprise several discrete toothed portions each meshing with one of the pinions 19 or with one of the just mentioned gear segments.

If the tube 1 is to be formed with relatively deep corrugations Cd with steep flanks of the type shown in FIG. 7, the grooving or corrugating unit preferably comprises a holder 302 which is a planetary having several equidistant planet pinions 24. These pinions are mounted on shafts 21 installed in a planet carrier 302a and mesh with a sun wheel 55 mounted on the shaft 304. The tools 303 are mounted on the pinions 24 in such a way that their planes are normal to the axis 5 and to the external surface 7 of the tube 1. The entire planetary 302 orbits about the axis of the tube when the apparatus is in use. The angular position (orientation) of the tools 303 remains unchanged when the planet pinions 24 travel about the sun wheel 55 either in response to lengthwise movement of the tube 1 or in response to starting of separate drive means for the planetary.

FIG. 8 illustrates a tube 1A having circumferentially complete corrugations C which are not equidistant from each other. Such formation can be achieved, for example, by driving the holder 2 of FIG. 1 at a variable speed or by mounting the tools at unequal distances from each other. FIG. 9 illustrates a second tube 18 having groups of closely adjacent circumferentially complete corrugations C whereby the corrugations of each group may but need not be exactly equidistant from each other. Also, the distance between successive groups may but need not be the same. Such method of corrugating can be employed when different sections of the finished product can or should exhibit different flexibility and/or when the non-corrugated portions 56 of the product are to be used for mounting of certain components, such as armatures, cable heads or the like. FIG. 10 shows a further tube 1D wherein the distance between the circumferential corrugations is nonuniform and the depth and/or width of corrugations also varies to a desired extent. Such configuration renders it possible to impart a desired flexibility to each of several successive sections of the product. The grooves or corrugations shown in FIG. 10 are formed with a unit including differently configurated and dimensioned tools which are mounted at different distances from each other.

FIG. ll shows a further tube wherein circumferentially complete corrugations alternate and/or are combined with helical and axially parallel corrugations. Such helical and axially parallel corrugations can be formed with angularly adjustable tools of the type shown in FIGS. 5 and 6. in the grooving unit of FIG. 6, the tools 203 are tumable through 90" so that they can be used to form circumferentially complete corrugations, helical corrugations or axially parallel grooves. The tools in a grooving unit (e.g., in the unit of FIG. 4) can be mounted in such a way that the distances between successive corrugations or groups of corrugations vary at regular intervals.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features which fairly constitute essential characteristics of the generic and specific aspects of our contribution to the art and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. A method of forming corrugations or analogous grooves in tubes consisting of ductile material, particularly in tubes for use as sheaths in electric cables, comprising the steps of moving a tube lengthwise along a predetermined path and univer sally movable guiding two longitudinally spaced portions of the tube so that the whole section of the tube between said portions can bend in direction transverse to its elongation; and orbiting about such section of the tube a plurality of grooving tools in such a position and at such a distance from said path that the whole section is bent in transverse direction and then grooved by successive tools, always by one tool at a time, whereby such grooving will occur without appreciable stretching of the ductile material of the tube.

2. Apparatus for forming corrugations or analogous grooves in tubes consisting of ductile material, particularly in tubes for use as sheaths in electric cables, comprising means for advancing a tube lengthwise along an elongated path; a pair of mutuully spaced universally movable bearing means each surrounding a portion of said path and having passages for closely guiding the advancing tube whereby the whole section of the tube between such bearing means can bend in direction transverse to its elongation; and a grooving unit disposed between said bearing means and including holder means arranged to orbit about said path and a plurality of grooving tools supported by said holder means for movement about an axis which is at least substantially normal to said path, said tools being sufficiently close to said path to bend said section between said bearing means in said transverse direction while tilting said bearing means and to groove the bent tube between said bearing means and said tools being arranged to travel about said axis at such a rate that the tube is engaged by one tool at a time.

3. Apparatus as defined in claim 2, wherein said tools are provided with concave tube-engaging faces.

4. Apparatus as defined in claim 2, wherein each of said bearing means comprises a substantially spherical bearing.

5. Apparatus as defined in claim 2, wherein one of said bearing means is located ahead of the other bearing means, as considered in the direction of travel of the tube, and the tool which engages the section of a tube between said bearing means is nearer to said other bearing means.

6. Apparatus as defined in claim 5, wherein the distance between said last-mentioned tool and said other bearing means is at least 1.5 times and the distance between said lastmentioned tool and said one bearing means is at least 2.5 times the diameter of the tube.

7. Apparatus as defined in claim 2, wherein said tools have concave tube-engaging faces and wherein the radii of curvature of such faces exceed the radius of the tube.

8. Apparatus as defined in claim 7, wherein each of said radii of curvature is between 1.1 and 1.8 times the radius of the tube.

9. Apparatus as defined in claim 2, wherein said tools are equidistant from each other and are disposed in planes including said axis so that each tool provides the travelling tube with a circumferentially complete corrugation, the maximum diameter of said corrugating unit being equal to 2 t= rfi cos a) wherein s is the distance between the centers of successive corrugations in the tube, r is the maximum depth of such corrugations, r is the radius of curvature of the profile on each tool, and alpha is the angle between the planes of adjoining tools.

10. Apparatus as defined in claim 2, further comprising adjusting means for moving said holder means substantially radially of said path.

11. Apparatus as defined in claim 10, further comprising a driven carriage supporting said holder means for orbital movement about said path, said adjusting means comprising a feed screw rotatably mounted in said carriage and arranged to move the holder means radially of said path in response to rotation thereof, and motor means for rotating said feed screw.

12. Apparatus as defined in claim 10, further comprising a rotary carriage supporting said holder means and movable therewith about said path, and a counterweight mounted on said carriage diametrically opposite said holder means, said adjusting means being arranged to move said counterweight in response to movement of said holder means radially of said path.

13. Apparatus as defined in claim 2, wherein said holder means comprises a hub having peripheral sockets for said tools and end walls connected with said hub and arranged to releasably hold the tools in the respective sockets.

14. Apparatus as defined in claim 2, wherein each of said tools is mounted for swiveling movement about an axis which is normal to said first-mentioned axis and wherein said holder means comprises meam for swiveling said tools about such axes.

15. Apparatus as defined in claim 14, wherein said holder means comprises a hub having peripheral sockets receiving said tools with clearance, said means for swiveling comprising end walls connected with said hub and arranged to retain said tools in the respective sockets, at least one of said end walls being arranged to turn about said first-mentioned axis to thereby swivel said tools.

16. Apparatus as defined in claim 14, wherein said means for swiveling comprises pinions provided on said tools and a gear rotatable about said first-mentioned axis and meshing with said pinions.

17. Apparatus as defined in claim 2, wherein said holder means comprises means for maintaining said tools in planes which are at least substantially normal to said path.

18. Apparatus as defined in claim 17, wherein said holder means comprises a planetary including a sun wheel rotatable about said axis, a planet carrier, and planet pinions mounted in said planet carrier and meshing with said sun wheel, said carrier and said pinions constituting said means for maintaining the tools in said planes.

[9. Apparatus as defined in claim 2, wherein said tools include at least two differently dimensioned tools.

20. Apparatus as defined in claim 2, wherein said tools are supported by said holder means at dilferent distances from each other to form in a travelling tube grooves at different distances from each other.

21. Apparatus as defined in claim 2, wherein said tools are disposed in planes making different angles with the longitudinal direction of said path.

22. Apparatus as defined in claim 2, wherein at least some of said tools are provided with lubricant-discharging ducts.

23. Apparatus as defined in claim 2, further compr'sing at least one lubricant-discharging nozzle arranged to orbit with said holder means and to lubricate such portions of a travelling tube which are grooved by said tools.

24. Apparatus as defined in claim 23, wherein said unit further comprises a counterweight arranged to orbit about said path and disposed diametrically opposite said holder means, said nozzle means being supported by said counterweight.

l I t i 

1. A method of forming corrugations or analogous grooves in tubes consisting of ductile material, particularly in tubes for use as sheaths in electric cables, comprising the steps of moving a tube lengthwise along a predetermined path and universally movable guiding two longitudinally spaced portions of the tube so that the whole section of the tube between said portions can bend in direction transverse to its elongation; and orbiting about such section of the tube a plurality of grooving tools in such a position and at such a distance from said path that the whole section is bent in transverse direction and then grooved by successive tools, always by one tool at a time, whereby such grooving will occur without appreciable stretching of the ductile material of the tube.
 2. Apparatus for forming corrugations or analogous grooves in tubes consisting of ductile material, particularly in tubes for use as sheaths in electric cables, comprising means for advancing a tube lengthwise along an elongated path; a pair of mutually spaced universally movable bearing means each surrounding a portion of said path and having passages for closely guiding the advancing tube whereby the whole section of the tube between such bearing means can bend in direction transverse to its elongation; and a grooving unit disposed between said bearing means and including holder means arranged to orbit about said path and a plurality of grooving tools supported by said holder means for movement about an axis which is at least substantially normal to said path, said tools being sufficiently close to said path to bend said section between said bearing means in said transverse direction while tilting said bearing means and to groove the bent tube between said bearing means and said tools being arranged to travel about said axis at such a rate that the tube is engaged by one tool at a time.
 3. Apparatus as defined in claim 2, wherein said tools are provided with concave tube-engaging faces.
 4. Apparatus as defined in claim 2, wherein each of said bearing means comprises a substantially spherical bearing.
 5. Apparatus as defined in claim 2, wherein one of said bearing means is located ahead of the other bearing means, as considered in the direction of travel of the tube, and the tool which engages the section of a tube between said bearing means is nearer to said other bearing means.
 6. Apparatus as defined in claim 5, wherein the distance between said last-mentioned tool and said other bearing means is at least 1.5 times and the distance between said last-mentioned tool and said one bearing means is at least 2.5 times the diameter of the tube.
 7. Apparatus as defined in claim 2, wherein said tools have concave tube-engaging faces and wherein the radii of curvature of such faces exceed the radius of the tube.
 8. Apparatus as defined in claim 7, wherein each of said radii of curvature is between 1.1 and 1.8 times the radius of the tube.
 9. Apparatus as defined in claim 2, wherein said tools are equidistant from each other and are disposed in planes including said axis so that each tool provides the travelling tube with a circumferentially complete corrugation, the maximum diameter of said corrugating unit being equal to , wherein s is the distance between the centers of successive corrugations in the tube, t is the maximum depth of such corrugations, r is the radius of curvature of the profile on each tool, and alpha is the angle between the planes of adjoining tools.
 10. Apparatus as defined in claim 2, further comprising adjusting means for moving said holder means substantially radially of said path.
 11. Apparatus as defined in claim 10, further comprising a driven carriage supporting said holder means for orbital movement about said path, said adjusting means comprising a feed screw rotatably mounted in said carriage and arranged to move the holder means radially of said path in response to rotation thereof, and motor means for rotating said feed screw.
 12. Apparatus as defined in claim 10, further comprising a rotary carriage supporting said holder means and movable therewith about said path, and a counterweight mounted on said carriage diametrically opposite said holder means, said adjusting means being arranged to move said counterweight in response to movement of said holder means radially of said path.
 13. Apparatus as defined in claim 2, wherein said holder means comprises a hub having peripheral sockets for said tools and end walls connected with said hub and arranged to releasably hold the tools in the respective sockets.
 14. Apparatus as defined in claim 2, wherein each of said tools is mounted for swiveling movement about an axis which is normal to said first-mentioned axis and wherein said holder means comprises means for swiveling said tools about such axes.
 15. Apparatus as defined in claim 14, wherein said holder means comprises a hub having peripheral sockets receiving said tools with clearance, said means for swiveling comprising end walls connected with said hub and arranged to retain said tools in the respective sockets, at least one of said end walls being arranged to turn about said first-mentioned axis to thereby swivel said tools.
 16. Apparatus as defined in claim 14, wherein said means for swiveling comprises pinions provided on said tools and a gear rotatable about said first-mentioned axis and meshing with said pinions.
 17. Apparatus as defined in claim 2, wherein said holder means comprises means for maintaining said tools in planes which are at least substantially normal to said path.
 18. Apparatus as defined in claim 17, wherein said holder means comprises a planetary including a sun wheel rotatable about said axis, a planet carrier, and planet pinions mounted in said planet carrier and meshing with said sun wheel, said carrier and said pinions constituting said means for maintaining the tools in said planes.
 19. Apparatus as defined in claim 2, wherein said tools include at least two differently dimensioned tools.
 20. Apparatus as defined in claim 2, wherein said tools are supported by said holder means at different distances from each other to form in a travelling tube grooves at different distances from each other.
 21. Apparatus as defined in claim 2, wherein said tools are disposed in planes making different angles with the longitudinal direction of said path.
 22. Apparatus as defined in claim 2, wherein at least some of said tools are provided with lubricant-discharging ducts.
 23. Apparatus as defined in claim 2, further comprising at least one lubricant-discharging nozzle arranged to orbit with said holder means and to lubricate such portions of a travelling tube which are grooved by said tools.
 24. Apparatus As defined in claim 23, wherein said unit further comprises a counterweight arranged to orbit about said path and disposed diametrically opposite said holder means, said nozzle means being supported by said counterweight. 